1. Field of the Invention
The present invention relates to load cells for measuring loads, comprising an elastic body fitted with sensor means to detect the strain or the deformation of the elastic body in response to the load to be measured.
The invention relates more specifically to a precision load cell comprising an elastic body where the elastic body comprises a first beam and a second beam that is positioned opposite to the first beam, a base end and a load receiving end that is positioned opposite the base end where the first and the second beams are connected to the base end and to the load receiving end via flexure points that provide sections for elastic deformation, further a sealed cavity comprising a flexible wall and sensor means for measuring the elastic deformation of the elastic body in response to the load to be measured.
2. Description of the Prior Art
Numerous sensor types are designed with sensor means adapted in a sealed cavity with the sensor means activated by a lever brought through a flexible wall in the sealed cavity. The flexible wall protects the sensor means from humidity and corrosion. Examples are joy sticks and simple force sensors, but precision measurements are in prior art only achieved by precision load cells provided with a parallelogram. Patent EP1451538, describes a precision load cell with sensor means mounted in a sealed cavity placed in the base end of the load cell.
The load cell, according to the patent EP1451538 does in practice display some problems.
One problem arises from temperature induced distortions of the walls of the sealed cavity for example when the rays of the sun shines on one of the walls, increasing the dimensions of this wall and hereby distorting the sealed cavity and influencing the sensor means, which give a transient shift of the zero. Because of the comparably extended geometry of the cavity with an appreciable distance between the walls, temperature equalization will not be complete, take a certain time and the zero signal will therefore only partly, and then slowly return to the correct value.
A further problem is the temperature rise above ambient temperature of the sensor means, caused by the power generated by the electronic signal processing circuits, which normally are mounted in the base end of the load cell near the sensor means. This temperature rise is difficult to compensate for in the zero and slope of the signals of the load cell as it is added to the temperature of the environment.
Another problem is the changing distortion of the cavity and the sensor means through a changing eccentricity of the loads applied on the load cell, which changes the forces in the two beams of the parallelogram. Because the beams are directly hinged on the sealed cavity, the changing forces in the beams will result in varying deformations of the walls of the sealed cavity and hereby influence the sensor means. This changing deformation of the walls of the sealed cavity, with changing eccentricity of the applied loads reduces the obtainable accuracies of the load cell.
Still another problem is the distortion of the sealed cavity and the sensor means when the mounting screws in the base end are tightened for mounting the load cell.
US Patent Application Publication No. US 2003/0111277 describe a number of precision load cells with sensor means placed in a sealed cavity in the base end of the load cell and with a lever connecting a flexible wall of the sealed cavity in the base end to the load receiving part.
The load cells according to US 2003/0111277 display the same problems as the load cell according to EP1451538.
U.S. Pat. No. 5,052,505 and US Patent Application Publication No. US 2011/0232393, both describe load cells where a sealed cavity for mounting sensor means is created by milling a recess in one of the beams of the parallelogram and closing the cavity with a cover.
Load cells according to these patents do both display a problem common to this type of load cell where strain gages are applied to the flexure points of the parallelogram. When a load cell of this design is loaded, the strain gage at one flexure point of a beam will see a positive strain from the bending of this flexure point while a strain gage at the other flexure point of the same beam will see a negative strain. Besides the bending strains there will however also appear strains in the flexure points from the longitudinal forces in the direction of the beam. These forces and strains will have the same sign in both flexure points of a beam and should ideally cancel each other when the signal of the load cell is measured in the normal Wheatstone bridge coupling of the strain gages. Inaccuracies in the strain gages themselves and in the bonding of the strain gages will however result in measurement errors with changing eccentricity of the load applied to the load cell, and these errors are tedious and time consuming to correct with filing or grinding of the flexure points.
It is the object of the invention to provide load cells with sealed cavities with sensor means which are thermally insulated from other parts of the load cell and which are not influenced by variations in the forces in the beams when varying the eccentricity of the load applied to the load cell and which are not influenced by tightening the mounting screws for the load cell.
Therefore, a need exists for a new and improved load cell having an elastic body that can be used for measuring loads. In this regard, the present invention substantially fulfills this need. In this respect, the load cell having an elastic body according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in doing so provide an apparatus primarily developed for the purpose of measuring loads.